The instant invention relates to polarization modulation and, more particularly, to the significant reduction of the effect of this phenomenon in earth orbiting scanning systems.
In earth observing satellite systems having an optical scanning assembly operably associated with a radiometer (or other detector), the radiometer (or other detector) will be adversely affected by scene polarization if the scanner is polarization sensitive.
Some optical scanning earth observing systems use folding mirrors (i.e., mirrors which fold the optical path) in the optical train, and therefore are polarization sensitive. It is these mirrors and their reflective coatings which cause a polarization problem. These mirrors constitute an analyzer, if the scene being viewed is polarized; and, scene polarization can occur when the scanner is viewing earth scenes which include clouds, or ice, or fog, or water, and the like. The polarized scene becomes the polarizer, and cross polarization occurs. As a result, unwanted amplitude modulation of the received signal (i.e., the beam of the image) occurs.
The simple solution is to add a depolarizer in the optical train of the scanner. However, no depolarizer presently exits that would be practical for such an optical system operating in orbit.
It is the reflecting surfaces of the folding mirror which make the scanner polarization sensitive. In this regard it is to be noted that the coatings most commonly used to make the mirror surfaces reflective are aluminum and silver. Test results establish that, on the average, aluminum mirrors polarize about thre times that of the silver mirrors. More specifically, in a folding mirror assembly which consists of three mirrors, such as will be discussed later herein, the signal modulation may vary as much as 6 percent if the reflective coatings are of aluminum, and as much as 1.5 percent if the reflective coatings are of silver. In a folding mirror assembly which consists of four mirrors, such as will be discussed later herein, the signal modulation may vary as much as 9 percent if the reflective coatings are of aluminum, and as much as 2 percent if the reflective coatings are of silver.
Additionally, these optical scanning systems typically are designed such that the normals (of the angles of incidence and reflection) to the reflecting surfaces of the mirrors are in the same plane. Unfortunately, this restraint adds to polarization, since the polarization is additive, as will be shown later herein.
As a result of all of the foregoing, the crossed polarization effect occurs, and the radiometer (or other detector) sees unwanted sinusoidal signal modulation.
Accordingly, what is needed in the art and is not presently available is some way to minimize the polarization modulation effect of these optical scanning systems that have a plurality of optically aligned, rotatable, flat folding mirrors which have incident angle normals in the same plane, which have aluminum or silver coated reflecting surfaces, and which serially reflect an image acquired by the scanning system while in orbit.